The present invention relates to the field of optical communications systems and in particular to a method for upgrading such systems.
Optical communications systems are a substantial and fast-growing constituent of communications networks. The expression xe2x80x9coptical communications systemxe2x80x9d, as used herein, relates to any system which uses optical signals to convey information across an optical medium. Such optical systems include, but are not limited to, telecommunications systems and local, metropolitan and wide area networks (LANs, MANs and WANs). Optical systems are described in Gowar, Ed. Optical Communications Systems, (Prentice Hall, N.Y.). Currently, the majority of optical communication systems are configured to carry only a single optical channel. In order to signal over this optical channel, a suitably modulated laser is used that emits light in a spectrum the center wavelength of which is not precisely defined (so-called xe2x80x9cgreyxe2x80x9d light). As a result the channel is allocated a relatively broad spectral band. The optical guide is specified to have a sufficiently low attenuation over this relatively broad spectral band to ensure acceptable signal to noise at the receiver. Here xe2x80x9coptical guidexe2x80x9d is used to describe any suitable optical transmission medium, including optical fibres and optical waveguides.
There is a need to access at nodes of the communications system, which may be comprised of, for example, rings, interconnected rings or meshes, the information carried in such optical guides so that individual messages may be routed to the correct destinations. To this end each node will contain switching circuitry. An economical form of switching circuitry for such nodes is the add-drop multiplexer (ADM). An ADM provides low cost access to all or part of the time division multiplexed (TDM) traffic forming a data stream passing along a communications link, such as an optical guide. The traffic passing through the ADM does so via xe2x80x9cline portsxe2x80x9d connecting to the bearer. Data or messages passing along the telecommunications bearer are selectively time division demultiplexed by switching circuitry in the ADM and the selected messages or message parts are passed via so-called tributary ports to their destination. Similarly, data or messages for adding to the telecommunications bearer are fed to the ADM via the tributary ports and are time division multiplexed into the message stream by the ADM switching circuitry. This switching and multiplexing function is performed in the electrical domain. In order to interface to an optical communications link, the nodes include optical to electrical converters (i.e. photo-detectors) and electrical to optical converters (i.e. laser signal generators).
The continuing and rapid increase in the amount of data traffic carried by telecommunications operators has led to an increasing need to improve the data carrying ability of existing networks. A conventional TDM communications system may be upgraded purely by increasing the TDM data rate of components of the link. This may call for the electronics equipments to be modified but permits the installed fibre link to be retained and used, thereby saving costs of additional fibre installation (which may be considerable). Current technology imposes strict limits on the gains available from increasing the TDM data rate due to optical sensitivity limitations in the receiver, safety limits to protect, e.g., maintenance workers, which restrict the amount of launch power which can be used, chromatic dispersion causing xe2x80x9ceye-closurexe2x80x9d in the digital signal at the receiver and other propagation impairments which depend upon TDM signaling rate.
The optical communications link. typically comprises optical fibre. The capacity of a dual fibre line system (in which one fibre is used to carry traffic in each direction) can be increased by introducing xe2x80x98single fibre workingxe2x80x99 into each of the fibres. Conventionally traffic flows in the fibre in one direction only. By introducing a directional optical coupler e.g. a fused twisted optical fibre pair coupler, at each end of a fibre, signalling may be effected in both directions on the one fibre. Thus the traffic previously passing on two fibres can be compressed onto one fibre and the second fibre then used as a separate line. This system is impaired by back scattering at the launch components and in the fibre itself and by beating between the signals from the two transmitter lasers (one at each end) and can be improved by selecting lasers with disparate wavelengths. Improved performance also accrues if the signalling wavelengths are chosen such that the receivers can be designed to be insensitive to the xe2x80x98otherxe2x80x99 wavelength. This latter procedure was the start of using wavelength selection towards capacity upgrade of fibre systems which has led on to wavelength division multiplexing discussed below.
The capacity of a single fibre line system can be increased by means of wavelength division multiplexing. In a wavelength division multiplexed (WDM) system there is a plurality of optical signals each signal having a spectrum whose center wavelength is constrained within a narrow spectral band, the acceptable pass band of the fibre being divided up into a multiplicity of such narrow spectral bands. By upgrading a single channel system to a two-channel WDM system i.e. replacing the single xe2x80x9cgreyxe2x80x9d channel with two narrow band channels its capacity may be increased. Whilst the TDM up-grade potential is strictly limited, the use of multi-channel WDM has a much greater potential for increased data rates with 8, 16, 32 and more channel WDM systems being proposed.
However, replacing a single channel photonic system with a WDM system supporting many different channels requires the replacement of a considerable amount of equipment at each node with new WDM equipment. This is expensive and may involve considerable over-provision of capacity bearing in mind that a doubling of capacity is often all that is required of a particular upgrade. There is therefore a need for a method for incrementally upgrading the traffic handling capacity of an optical communications system.
The present invention provides a method for upgrading an optical communications system; in which the system comprises a plurality of nodes of which two are linked by a single optical path for the communication, from the node at a first end of the optical path to the node at the other end of the optical path, of traffic comprising a single optical channel; in which each node comprises add/drop multiplex (ADM) means for adding and dropping signals in electrical form, in which the node at the first end of the optical path comprises a single light source for converting from electrical to optical form signals output by the ADM means for transmission via the optical path; and in which the node at the other end of the optical path comprises a photodetector for converting signals received via the optical path for input to the ADM means; the method including the steps of replacing the single light source at the first end of the optical path with a plurality of stable narrowband light sources having mutually different wavelengths, installing at the first end of the optical path optical multiplex means for multiplexing the outputs of the plurality of stable narrowhand light sources into the optical path; replacing the single photodetector at the other end of the optical path with a plurality; installing at the other end of the optical path optical demultiplex means for demultiplexing a like plurality of received optical channels, whereby each of the plurality of optical channels is converted to electrical signals and applied to ADM means.
The present invention also provides a method for upgrading an optical communications system; in which the system comprises a plurality of nodes linked by one or more optical paths, each path for the communication of traffic comprising a single optical channel; in which each node comprises add/drop multiplex (ADM) means for adding and dropping signals in electrical form, and conversion means for converting between electrical and optical form signals output by the ADM means for transmission via a first one of the optical paths; and for converting signals received via a second one of the optical paths for input to the ADM means; the method comprising the steps of installing optical demultiplex means for selecting signals received via the second one of the optical paths according to wavelength for input to the ADM means; installing means for converting the signals output by the ADM means into a first stable narrow wavelength band optical signal; and installing optical multiplex means for guiding the first signal into the first optical path and comprising means for allowing one or more further stable narrow wavelength band optical signals having different wavelengths from the first signal to be added and combined with the first signal into the first optical path.
According to a preferred embodiment, the present invention provides a method comprising switching individual time division multiplexed messages between wavelength multiplexed channels of the optical communications system, comprising the steps of arranging switch means comprising a plurality of add drop multiplex (ADM) means, one per wavelength multiplex channel; in which each ADM means comprises tributary means, the method comprising the steps of interconnecting the ADMs means via the tributary means.
According to a preferred embodiment the present invention provides a method comprising switching individual time division multiplexed messages between a plurality of streams of time division multiplexed data; in which each stream of time division multiplexed data is comprised in a channel of the wavelength multiplexed optical communications system, the method comprising the steps of providing one or more of the nodes with a plurality of add drop multiplex (ADM) means, one per WDM signal; providing each ADM means with tributary means and interconnecting the ADM means via the tributary means.